The present invention relates to an extractant effective in selectively extracting and removing strontium (Sr) from an aqueous solution containing the same.
A high-level radioactive liquid waste generated in the step of reprocessing a spent nuclear fuel contains radioactive isotopes which have long half-lives or an exothermic property, thereby posing problems in the processing and disposal of the liquid waste. It is demanded to selectively remove Sr, which is particularly an exothermic nuclide having a long half-life among the radioactive isotopes.
Investigations have been made hitherto on solvents capable of selectively extracting a metal ion from an aqueous solution containing the same. Among them, for example, an extractant having a capability of causing structure-function transformation is known. As shown in FIG. 2, this extractant can, merely by constituting it as a linear chain compound having neutral ligands (N) and, in addition, anionic ligands (A) at its terminals, induce a structural change by utilizing the coordination of the anionic ligands to the first transition metal (M.sub.1) to change its structure from the linear one to a cyclic one, thereby creating a new molecular arrangement for accepting the second metal ion (M.sub.2) in cooperation with the neutral ligands.
Therefore, the extractant having the capability of causing structure-function transformation has features such that it can be synthesized more easily than the conventional cyclic compounds such as crown ethers, that it can capture various metal ions, since both electronical and conformational nature of the site can be changed by changing M.sub.1, and that it can control the incorporation and release of M.sub.1 by the oxidation and reduction of M.sub.1 itself, thus facilitating the capture and release of M.sub.2.
However, among the extractants having the above-described capability, no extractant particularly effective in selectively extracting Sr has been synthesized hitherto.